This invention relates to a bass reproduction speaker apparatus that provides improved bass reproduction performance even if the cabinet is small.
Regarding the bass reproduction of a typical speaker, there is an inverse proportional relationship between the cabinet internal volume V, bass reproduction limit frequency fc, and efficiency xcexc. Therefore, as commonly known, it is very difficult to reproduce lower frequencies efficiently in a small cabinet.
It has been also known that bass reproduction performance can be improved without concern for these constraints if negative stiffness is used to decrease air stiffness in the cabinet and increase equivalently the internal volume of the cabinet. Actually, however, there have been no suitable methods to achieve this purpose.
The following are conventional techniques disclosed to realize the concept in U.S. Pat. No. 2,810,021 patented on Oct. 15, 1957 (Low Frequency Loudspeaker), and in U.S. Pat. No. 4,607,382 patented on Aug. 19, 1986 (Electroacoustic Transducer Unit With Reduced Resonant Frequency And Mechanical Spring With Negative Spring Stiffness, Preferably Used In Such A Transducer Unit).
FIG. 17 shows a configuration of a conventional bass reproduction speaker apparatus disclosed in U.S. Pat. No. 2,810, 021.
In FIG. 17, 351 denotes a speaker unit, and the speaker unit 351 includes a field magnetic portion 351a, a frame 351b, a voice coil 351c, a damper 351d, an edge 351e and a diaphragm 351f. Numeral 352 denotes an airtight cabinet to which the speaker unit 351 is attached.
Numeral 358 denotes a supporter that is fixed to the cabinet 352. Springs 359b disposed on the inner wall of the supporter 358 press levers 359c in an inward direction. The levers 359c are supported by fulcrum grooves 359d. A rod 359a is attached to the upper part of the voice coil 351c of the speaker unit 351, and toggle pins 359f are entrapped between grooves 359g of the rod 359a and grooves 359e of the levers 359c. 
A movable electrical contact 360a is provided at the upper part of the rod 359a, and the contact 360a is flexibly supported by a spring 360c. And a fixed electrical contact 360b is arranged to sandwich the movable electrical contact 360a. To these electrical contacts, an exhaust pump 360d and an intake pump 360e are connected.
A conventional bass reproduction speaker apparatus thus configured operates as follows.
The springs 359b press the toggle pins 359f in the inward direction through the levers 359c. Therefore, when the voice coil 351c is displaced and the toggle pins 359f lose the equilibrium, the toggle pins 359f are tilted further and press the rod 359a in the displacement direction.
When a vibration system including the voice coil 351c and the diaphragm 351f is displaced, the stiffness of a supporting system (the damper 351d and the edge 351e) and the stiffness of air in the cabinet 352 act to pull the vibration system back to the central position. However, the toggle mechanism including 359a-359g generates a force in the reverse direction, more specifically, the toggle mechanism acts to further push out the vibration system of the speaker unit 351 in the displacement direction.
In other words, the toggle mechanism including 359a-359g provides negative stiffness to the vibration system of the speaker unit 351. Since the toggle mechanism cancels and reduces the stiffness provided by the supporting system of the speaker unit 351 and of the air in the cabinet 352, the internal volume of the cabinet 352 is increased equivalently. This results in improvement of the bass reproduction performance. The principle of the equivalent increase of the cabinet internal volume due to negative stiffness and improvement in the bass reproduction performance will be described later in detail.
When this negative stiffness is greater than the stiffness of the supporting system of the speaker unit 351, the vibration system cannot stay at the inherent displacement central position due to a slight air leak from the cabinet 352 or the like, but it is offset in either displacement direction. The electrical contacts (360a, 360b) and the pumps (360d, 360e) serve to correct the offset.
More specifically, when the vibration system of the speaker unit 351 is offset forward, the movable electrical contact 360a contacts with the upper part of the stationary electrical contact 360b, and the exhaust pump 360d activates. As a result, air in the cabinet 352 is exhausted and the vibration system of the speaker unit 351 is pulled back to the inherent displacement central position. On the contrary, when the vibration system is offset backward, the movable electrical contact 360a contacts with the lower part of the stationary electrical contact 360b, and the intake pump 360e activates. As a result, air flows into the cabinet 352, and the vibration system of the speaker unit 351 is pushed back to the inherent displacement central position.
Sequentially, an offset in the displacement direction of the vibration system of the speaker unit 351 is corrected even when the negative stiffness is great.
However, the springs 359b in the above-mentioned configuration will have a mechanical fatigue since the negative stiffness is generated by the mechanical toggle mechanism including 359a-359g. Operation with large amplitude for a long time can cause a rupture. This will deteriorate the reliability. Moreover, the portions at which the toggle pins 359f and the levers 359c contact with each other generate abnormal noises. In addition, increased numbers of components make the apparatus complicated.
The apparatus inevitably will be more complicated and large-scaled since it requires pumps (360d, 360e) to correct an offset in the displacement direction of the vibration system, and the pumps cause noises in a case that negative stiffness is great.
Configurations of other conventional bass reproduction speaker apparatuses disclosed in U.S. Pat. No. 4,607,382 are shown in FIGS. 18 and 19. These speaker apparatuses are further developed though the basic principles thereof are identical to the first speaker apparatus.
In FIGS. 18 and 19, 451 denotes an electrodynamic speaker unit, and it includes a field magnetic portion 451a, a frame 451b, a voice coil 451c, a damper 451d, an edge 451e, and a diaphragm 451f. Numeral 452 denotes an airtight cabinet to which the speaker unit 451 is attached.
Numeral 451i denotes a ring to reinforce the diaphragm 451f, and the ring is attached to the outer rim of the diaphragm 451f. Numeral 454 denotes pairs of springs respectively composed of two warped plate springs opposing each other. While being compressed in the longitudinal direction, one end of each spring 454 is attached to a movable part supporting member 451g on the reinforcement ring 451i, and the other end is attached to the stationary part supporting member 452a that is fixed to the frame 451b. Two springs 454 are arranged longitudinally in a line centering the movable part supporting member 451g. In this example, three sets of spring pairs 454 arranged in a line are provided to be substantially rotationally symmetric about the central axis of the diaphragm 451f. 
Two springs 454 should be arranged in a line on both sides of the movable part supporting member 451g. If there is only one pair of springs 454, a rotational force about the central axis will act on the reinforcement ring 451i (i.e., the diaphragm 451f, and thus, the supporting systems of the speaker unit 451 are subjected to stress and can be damaged.
Numeral 458 denotes a means to detect displacement of the diaphragm 451f from the central position of the vibration. U.S. Pat. No. 4,607,382 refers to detection methods including capacitive detection, inductive detection, optoelectrical detection, and pneumatic detection.
Numeral 459 denotes a controller to correct displacement of the diaphragm 451f from the central position for vibration, and it operates based on an output signal from the detector 458. In FIG. 18B, 460 denotes an intake-exhaust pump, which operates based on an output signal from the controller 459. In FIG. 19B, the controller 459 is a high-power amplifier provided aside from an ordinary power amplifier that amplifies a source signal, and the controller 459 is connected with the speaker unit 451.
Operations of the conventional bass reproduction speaker apparatus thus configured are explained below, by further referring to FIGS. 20 and 21.
FIGS. 20 and 21 show the action of negative stiffness provided by the springs 454. In FIG. 20, 452a denotes a stationary part supporting member, 451g denotes a movable part supporting member and 454 denotes a pair of springs attached with a compressive force of the supporting members. In other words, FIG. 20 shows a pair of springs 454 in FIGS. 18 and 19.
When the movable part supporting member 451g is positioned at the center in the vibration displacement direction x(x=0), repulsion of the springs 454 acting between the movable part supporting member 451g and the stationary part supporting member 452a is directed perpendicular to the x direction, and a vector component of the force in the x direction becomes zero. As a result, no forces in the x direction are generated.
However, when the position of the movable part supporting member 451g is displaced from the center, i.e., when the movable part supporting member 451g moves, for example, to the xcex94x position in FIG. 20, the direction of the repulsion of the springs 454 acting between the movable part supporting member 451g and the stationary part supporting member 452a is not perpendicular to the x direction. As a result, a vector component of a force in the x direction is generated and a force to push the movable part supporting member 451g in the xcex94x direction is generated.
Here, F1 indicates a force applied to the vibration system of the speaker unit 451 by the supporting system of the speaker unit (e.g., the edge 451e) and by the air enclosed in the cabinet 452 when the movable part supporting member 451g is displaced by xcex94x. In such a case, F1 is a force to pull the movable part supporting member 451g back to the position of x=0, so apparently, the polarity of the stiffness of the force F1 is positive.
Similarly, F2 indicates a force applied to the vibration system by the springs 454 when the movable part supporting member 451g is displaced by xcex94x. In such a case, F2 is a force to further push the movable part supporting member 451g in the displacement direction. Therefore, the direction of F2 is reverse to the direction of F1, and the polarity of the stiffness of the force of F2 is negative.
In this way, the springs 454 generate negative stiffness. The action of this negative stiffness is shown in FIG. 21. In FIG. 21, the broken line indicates the relationship between an x direction displacement and a force that positive stiffness of the speaker unit supporting system and of the air in the cabinet provides to the vibration system, i.e., the movable part supporting member 451g. The dashed line indicates the relationship between the x direction displacement and a force that negative stiffness of the springs 454 provides to the movable part supporting member 451g. The solid line indicates the relationship between the x direction displacement and a force that the total stiffness including the above-identified stiffness provides to the movable part supporting member 451g. 
The positive stiffness is F1/xcex94x, and it corresponds to the gradient of the broken line. The negative stiffness is F2/xcex94x, and it corresponds to the gradient of the dashed line. The total stiffness is Ft/xcex94x, i.e. (F1xe2x88x92F2)/xcex94x, and it corresponds to the gradient of the solid line. The gradient of the solid line is smaller than that of the broken line, and this implies that the total stiffness is decreased. As mentioned above, stiffness finally applied to the movable part supporting member 451g is decreased due to the action of the negative stiffness, and the effect is equivalent to the case where air stiffness in the cabinet is decreased. Since the stiffness of the air in the cabinet is inversely proportional to the internal volume of the cabinet, the effect is equivalent to the case where the internal volume of the cabinet 452 is increased.
To obtain this effect sufficiently, the negative stiffness of the springs 454 should be increased. In this case, however, the supporting system of the speaker unit 451 yields to this negative stiffness, and the vibration system of the speaker unit 451 is completely offset to a position out of the displacement central portion of x=0, so that normal operations will be hindered.
Though the total stiffness composed of the stiffness of the air in the cabinet 452 and of the stiffness of the supporting system of the speaker unit 451 is greater than the negative stiffness, a trace of air is leaked inevitably from the cabinet 452 or from the diaphragm 451f. Therefore, if the negative stiffness exceeds the stiffness of the supporting system of the speaker unit 451, the vibration system cannot stay at the displacement central position.
To prevent and correct this, the detector 458, the controller 459 and the intake-exhaust pump 460 are provided as shown in FIGS. 18 and 19.
In the configuration exemplified in FIGS. 18A and 18B, when the average vibration displacement center of the vibration system of the speaker unit 451 is offset forward from the inherent central position (a position of x=0 in FIG. 20), the detector 458 detects this displacement offset and generates a signal to send an output signal to the controller 459. The controller 459 actuates the intake-exhaust pump 460 to exhaust the air in the cabinet 452, so that the diaphragm 451f of the speaker unit 451 is pulled back to its inherent displacement central position.
On the contrary, when the vibration system is offset backward, the intake-exhaust pump 460 is actuated to intake air into the cabinet 452, so that the vibration system of the speaker unit 451 is pushed back to its inherent displacement central position.
In the configuration shown in FIGS. 19A and 19B, when the vibration system of the speaker unit 451 is offset forward, the controller 459, i.e., a power amplifier, supplies current to the voice coil 451c of the speaker unit 451 in order to pull the vibration system back to its inherent displacement central position. On the contrary, when the vibration system is offset backward, the controller 459 supplies current in the inverse direction to the voice coil 451c of the speaker unit 451 in order to push the vibration system back to its inherent displacement central position.
In FIGS. 19A and 19B, the voice coil 451c should have a double-voice coil configuration composed of two voice coils, i.e. a voice coil to run a current of this controller 459 and an original voice coil to reproduce a source signal.
In the conventional bass reproduction speaker apparatuses configured as shown in FIGS. 18 and 19, the bass reproduction performance can be improved by increasing equivalently the internal volume of the cabinet by using negative stiffness while correcting an offset in the displacement direction of the vibration system of the speaker unit 451. Details of the principle of the equivalent increase of the cabinet internal volume due to negative stiffness and improvement in the bass reproduction performance are described later in the embodiments of the present invention.
However, in the above-mentioned configurations of the conventional techniques, the intake-exhaust pump or an additional power amplifier other than the power amplifier for source signal reproduction is required to correct an offset in the displacement direction of the vibration system of the speaker unit 451. As a result, the apparatuses will be complicated and large, and the cost will rise. Moreover, the springs 454 have a mechanic fatigue easily, resulting in poor reliability.
In conclusion, the bass reproduction speaker apparatuses disclosed in U.S. Pat. No. 2,810,021 and 4,607,382 are to increase equivalently the cabinet internal volume and to improve bass reproduction performance. However, these apparatuses suffer from many problems as mentioned above, and they are not practical.
The present invention aims to solve the above-mentioned problems of the conventional techniques by providing a bass reproduction speaker apparatus that is reliable, inexpensive, simple and practical. The bass reproduction speaker apparatus has excellent bass reproduction performance since the internal volume of the cabinet is increased equivalently.
For achieving this purpose, the present invention has the following configuration.
More specifically, a first bass reproduction speaker apparatus of the present invention includes a speaker unit having a vibration system, a cabinet to which the speaker unit is attached, a movable magnet that moves together with the vibration system of the speaker unit, and a stationary magnet, in which the movable magnet and the stationary magnet are configured to generate negative stiffness to the vibration system of the speaker unit. Accordingly, negative stiffness can be generated without using any mechanical means or any contacts, so that no mechanical fatigues or noises will occur. Therefore, a reliable, simple and practical bass reproduction speaker apparatus can be provided.
In the first bass reproduction speaker apparatus, it is preferable that the stationary magnet is ring-like and the movable magnet is arranged at the inner radius of the stationary magnet. Accordingly, only two magnets are required to generate negative stiffness, and thus, the mechanism for generating negative stiffness can be simplified. Furthermore, as the stationary magnet can be made bigger, extremely great negative stiffness can be obtained easily. Moreover, a characteristic curve of the displacement-force of the generated negative stiffness is linear. Therefore, a bass reproduction speaker apparatus that is simple, useful and excellent in the performance can be obtained.
It is preferable in the first bass reproduction speaker apparatus that the movable magnet and the stationary magnet are configured so that the generated negative stiffness is decreased before the displacement of the vibration system of the speaker unit reaches its maximum. Accordingly, the vibration system of the speaker unit is braked before the maximum amplitude is obtained. As a result, no abrupt tension will be applied to the supporting system in a case where excessive input is applied to the speaker unit, and a bass reproduction speaker apparatus resistant to excessive input is obtainable.
In the first bass reproduction speaker apparatus, it is preferable that a detector to generate a signal according to the displacement of the vibration system of the speaker unit is provided to feed back the signal from the detector to a power amplifier for driving the speaker unit in order to correct an offset in the displacement direction of the vibration system of the speaker unit. Accordingly, the speaker apparatus operates stably even when the generated negative stiffness is greater than the stiffness of the supporting system of the speaker unit. And thus, a bass reproduction speaker apparatus having further excellent bass reproduction performance can be obtained, with its equivalent internal volume of the cabinet being made extremely large.
Here, it is preferable that the detector includes a Hall element. Accordingly, the means to detect an offset in the displacement direction of the vibration system of the speaker unit can be simplified.
In the first bass reproduction speaker apparatus, it is preferable that the speaker apparatus has a holder to hold the vibration system of the speaker unit around the central position in the displacement direction when the speaker apparatus does not operate. Accordingly, the vibration system of the speaker unit is prevented from being offset to one side for a long time when the bass reproduction speaker apparatus does not operate, so that stress applied to the edge or the damper can be reduced. Therefore, a long-life bass reproduction speaker apparatus with less change over time can be provided.
In the first bass reproduction speaker apparatus, it is preferable that the stationary magnet is an electromagnet. Accordingly, the generated negative stiffness can be controlled by increasing or decreasing current running through the electromagnet. Sequentially, a bass reproduction speaker apparatus with variable fundamental resonance frequencies can be obtained. Moreover, the generation of the negative stiffness can be prevented by stopping the energizing of the electromagnet when the bass reproduction speaker apparatus does not operate, and thus, a long-life bass reproduction speaker apparatus with less change over time can be provided.
A second bass reproduction speaker apparatus includes a speaker unit having a vibration system, a means to provide negative stiffness to the vibration system of the speaker unit, a cabinet to which the speaker unit is attached, a detector to generate a signal according to displacement of the vibration system of the speaker unit, and a feedback circuit to feed back the signal from the detector to a power amplifier that supplies source signal power to drive the speaker unit. The power amplifier supplies power to the speaker unit in order to correct an offset in the displacement direction of the vibration system of the speaker unit along with source signal power. Accordingly, the speaker unit is servoed to constantly correct even a slight offset in the displacement direction of the vibration system and to hold the vibration system at its inherent central position in the displacement direction. As a result, the average displacement central position of the vibration system is held at its inherent central position even if the negative stiffness is great, and thus, extremely stable operation can be obtained. Furthermore, unlike the conventional apparatuses, there is no need to separately provide an intake-exhaust pump or a power amplifier for control in order to correct an offset in the displacement direction of the vibration system. Therefore, the present invention can provide a bass reproduction speaker apparatus having a simple and practical configuration at a low cost, and the apparatus has excellent bass reproduction performance.
In the second bass reproduction speaker apparatus, it is preferable that the detector includes a movable magnet that moves together with the vibration system of the speaker unit, and a Hall element to detect magnetism of the movable magnet. Accordingly, a single element can distinguish the rise and fall in displacement of the vibration system of the speaker unit. Therefore, a bass reproduction speaker apparatus with a simply configured detector is provided.
In the second bass reproduction speaker apparatus, the means to provide negative stiffness can be composed of springs that are attached at plural positions in a compressed state between the parts around the central position of the vibration system of the speaker unit and in the vicinity of the outer rim of the speaker unit or the cabinet, and the springs are attached with a substantial symmetry about a central axis. Accordingly, the springs can be made longer and the mechanical fatigue is reduced. Therefore, a bass reproduction speaker apparatus with improved reliability can be provided.
Alternatively in the second bass reproduction speaker apparatus, the means to provide negative stiffness can be composed of a movable magnet that moves together with the vibration system of the speaker unit and a stationary magnet to provide a force in the displacement direction of the vibration system to the movable magnet. Since the negative stiffness is generated without using any mechanical means, no mechanical fatigue will occur in this example. Therefore, a bass reproduction speaker apparatus with further improved reliability can be provided.
In the second bass reproduction speaker apparatus, it is preferable that a holder is provided to hold the vibration system of the speaker unit around the central position in the displacement direction during inoperative conditions. Accordingly, the vibration system of the speaker unit is prevented from being offset to one side in the displacement direction for a long time when the bass reproduction speaker apparatus does not operate, and thus, the supporting system and the springs of the speaker unit are not subjected to stress over a long time. Therefore, a long-life bass reproduction speaker apparatus with less change over time can be provided.
It is preferable that the holder includes a self-contained solenoid. Accordingly, the vibration system of the speaker unit can be held around the central position in the displacement direction and the holding is released in instantaneous operations. Therefore, a bass reproduction speaker apparatus that starts operating promptly is provided.
It is further preferable that a return current of the self-contained solenoid is supplied by discharging from a capacitor while an operation current of the self-contained solenoid is supplied through the same capacitor. Accordingly, the vibration system of the speaker unit can be held around the central position in the displacement direction even when the power supply is cut off suddenly during the operation of the bass reproduction speaker apparatus. Therefore, a bass reproduction speaker apparatus with improved safety can be provided.
In the second bass reproduction speaker apparatus, it is preferable that the negative stiffness provided by the above-mentioned means is reduced when the speaker apparatus does not operate. Accordingly, tension applied to the supporting system of the speaker unit during inoperative conditions can be reduced, and a long-life bass reproduction speaker apparatus with less change over time is provided.
In the second bass reproduction speaker apparatus, it is also preferable that the negative stiffness provided by the above-mentioned means is variable. Accordingly, the generated negative stiffness can be adjusted so that the fundamental resonance frequency of the bass reproduction speaker apparatus can be adjusted. Therefore, a bass reproduction speaker apparatus with a variable bass characteristic can be provided.
As mentioned above, the present invention provides a bass reproduction speaker apparatus with a simple configuration at a low cost. The apparatus is reliable and practical, and it has excellent bass reproduction performance. Therefore, the present invention has a great value from the viewpoint of utility.